Vacuum processes can be used to deposit organic and inorganic materials onto substrates. The vacuum deposition of metal coatings, for example, is widely used in the manufacture of various articles, ranging from jewelry to automobiles. Electronic and microelectronic manufacturing are particularly dependent upon vacuum deposition.
Recently, vacuum deposition processes have been proposed as a technically superior and less expensive alternative to wet chemical metallization techniques for medium technology manufacturing. In the past, wet chemical metallization has been used in medium technology manufacturing to deposit metals onto dielectric substrates (i.e., workpieces). Typically these substrates are formed as intermediate sized panels (e.g., 600.times.800 mm). One problem with wet chemical metallization for coating dielectric substrates is that it is an expensive process that requires surface pretreatment and catalyzation prior to metal deposition. In addition, wet chemical metallization processes involve toxic chemicals that must be used and disposed of in a manner that minimizes atmospheric and ground water pollution.
In general, vacuum deposition processes can be divided into three categories: vacuum evaporation, sputtering and ion plating. Vacuum evaporation is a thermal process carried out in a vacuum chamber containing a substrate and a gaseous deposition species. A heating element (e.g., filament, electron beam, hot plate) heats and vaporizes a solid material to form an evaporant cloud. Because of the proximity of the evaporant to the substrate and the influence of the vacuum, evaporant molecules strike the substrate and solidify to form a coating.
One shortcoming of vacuum evaporation processes is the lack of adhesion of the coating to the substrate. This is because the evaporant is formed by simply vaporizing a deposition species and therefore the evaporant particles possess a relatively low kinetic energy. Furthermore, there is no particle acceleration into the substrate and consequently the evaporant particles impact the substrate with very little momentum resulting in poor adhesion to the substrate. In addition, vacuum evaporation processes require a "line-of-sight" between the evaporant cloud and the substrate. Uniformity may be a problem because the quality and thickness of the deposited coating varies with the angle of incidence of the evaporant particles from the evaporant cloud to the substrate. In order to deposit coatings with a uniform thickness, the substrate must be rotated. Typically, this involves the use of transport mechanisms that support the substrate as it is moved in a rotary, linear or planetary, motion with respect to the source.
Sputtering, another type of vacuum deposition process, is a physical process that can be used to deposit a variety of metallic and dielectric materials onto a wide range of substrates. In sputtering, material is removed from a source or target (i.e., the cathode) by bombardment of a plasma. This material is deposited on the surface of the substrate which may be cathodic or floating with respect to the cathode. The plasma is formed of an inert gas, typically argon, and comprises mobile, positively and negatively charged particles. The plasma may be generated by an externally applied dc or rf field that supplies energy to the inert gas to remove an outer shell electron and produce separate positive and negative charged particles. Magnetic fields are also utilized to increase the effectiveness of the argon ions in removing material from the target (i.e., magnetron sputtering).
Sputtering is the most common alternative to vacuum evaporation. In general, sputtering improves the adhesion of the coating to a substrate because atoms extracted from the target by bombardment have more kinetic energy than those from an evaporation source. These energetic particles strike the substrate with sufficient energy to embed themselves in the substrate and to form a coating with good adhesive characteristics. A disadvantage of sputtering is that the inert gas which forms the plasma is often included in the deposited coating and may adversely affect the properties of the coating. In addition, deposition rates with sputtering are slower than vacuum evaporation and equipment is more expensive and complicated.
With ion plating, a third type of vacuum deposition process, an evaporant passes through a dc inert gas plasma, typically argon, and is ionized. A small fraction of ionized evaporant is accelerated to the surface of the substrate which is the cathode (negative electrode) of the dc plasma discharge. Typically a negative bias of several thousand volts with respect to the plasma is applied to the substrate. Ion plating, like sputtering, is not limited to line-of-sight deposition and provides superior adhesion over vacuum evaporation. In addition, for many applications, the substrate need not be rotated to achieve adequate coating uniformity and a wide variety of materials can be deposited.
Disadvantages of ion plating include its expense, the difficulty in masking parts from the deposition ions, and the damage to the substrate and coating from continuous bombardment by the accelerated ions.
Another vacuum deposition process termed "gasless ion plating" is a combination of evaporation and ion plating. Such a process is disclosed in U.S. Pat. Nos. 4,039,416; 4,420,386 and Re. 30,401 to Gerald W. White. With this deposition process, a plating material is vaporized and simultaneously subjected to ionization in a vacuum environment. The substrate is biased by an external dc source. A "virtual" cathode is created near the substrate using a rf power supply. The rf power supply is connected to the substrate to form a plasma of ionized atoms from the evaporant. The ionization during evaporation enhances deposition onto the substrate. This technique is termed "gasless" because the plasma is formed solely from the evaporant and does not require an inert gas to be maintained.
Although the vacuum deposition processes disclosed in the White patents provide enhanced deposition, the uniformity of the deposited coating over large areas is still a problem. Furthermore, the magnetic field configurations used at the substrate generate local regions on the substrate of non uniform deposition and increased heating. These nonuniformities will adversely effect the quality and properties of the deposited coating. Temperature sensitive substrates are particularly susceptible to the effects of localized heating.
In general, prior art vacuum deposition processes have not been successful in replacing wet chemical metallization processes for plating intermediate-sized workpieces. This failure is due, in part, to the lack of systems integration and optimization in vacuum process equipment that is available for the cost-driven, medium technology metallization market. Additionally, current vacuum metallization processes for dielectric workpieces require capital intensive processing equipment. Vacuum metallization equipment typically costs 5 to 10 times its wet chemical counterpart. In view of the foregoing, the need still exists for a method and apparatus of vacuum metallization which will economically satisfy medium technology manufacturing needs.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for vacuum deposition of highly ionized media in an electromagnetic controlled environment suitable for coating substrates with a high degree of uniformity and adhesion. It is a further object of the present invention to provide an improved method and apparatus for vacuum deposition of highly ionized media in an electromagnetic controlled environment that is adaptable to large scale, medium technology manufacturing. It is yet another object of the present invention to provide an improved method and apparatus for vacuum deposition of highly ionized media in an electromagnetic controlled environment that is less expensive and more efficient than prior art wet chemical metallization processes for intermediate-sized substrates.